Evidence in genetics

phylogenetic tree of life

One of the strongest evidences for common descent comes from the study of gene sequences. Comparative sequence analysis examines the relationship between the DNA sequences of different species, producing several lines of evidence that confirm Darwin's original hypothesis of common descent. If the hypothesis of common descent is true, then species that share a common ancestor inherited that ancestor's DNA sequence, as well as mutations unique to that ancestor. More closely related species have a greater fraction of identical sequence and shared substitutions compared to more distantly related species.

The simplest and most powerful evidence is provided by phylogenetic reconstruction. Such reconstructions, especially when done using slowly evolving protein sequences, are often quite robust and can be used to reconstruct a great deal of the evolutionary history of modern organisms (and even in some instances such as the recovered gene sequences of mammoths, Neanderthals or T. rex, the evolutionary history of extinct organisms). These reconstructed phylogenies recapitulate the relationships established through morphological and biochemical studies. The most detailed reconstructions have been performed on the basis of the mitochondrial genomes shared by all eukaryotic organisms, which are short and easy to sequence; the broadest reconstructions have been performed either using the sequences of a few very ancient proteins or by using ribosomal RNA sequence.

Phylogenetic relationships also extend to a wide variety of nonfunctional sequence elements, including repeats, transposons, pseudogenes, and mutations in protein-coding sequences that do not result in changes in amino-acid sequence. While a minority of these elements might later be found to harbor function, in aggregate they demonstrate that identity must be the product of common descent rather than common function.

DNA sequencing


Comparison of the DNA sequences allows organisms to be grouped by sequence similarity, and the resulting phylogenetic trees are typically congruent with traditional taxonomy, and are often used to strengthen or correct taxonomic classifications. Sequence comparison is considered a measure robust enough to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons. Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other apes. The sequence of the 16S ribosomal RNA gene, a vital gene encoding a part of the ribosome, was used to find the broad phylogenetic relationships between all extant life. The analysis, originally done by Carl Woese, resulted in the three-domain system, arguing for two major splits in the early evolution of life. The first split led to modern Bacteria and the subsequent split led to modern Archaea and Eukaryotes.

Endogenous retroviruses


Endogenous retroviruses (or ERVs) are remnant sequences in the genome left from ancient viral infections in an organism. The retroviruses (or virogenes) are always passed on to the next generation of that organism that received the infection. This leaves the virogene left in the genome. Because this event is rare and random, finding identical chromosomal positions of a virogene in two different species suggests common ancestry.

Proteins


The proteomic evidence also supports the universal ancestry of life. Vital proteins, such as the ribosome, DNA polymerase, and RNA polymerase, are found in everything from the most primitive bacteria to the most complex mammals. The core part of the protein is conserved across all lineages of life, serving similar functions. Higher organisms have evolved additional protein subunits, largely affecting the regulation and protein-protein interaction of the core. Other overarching similarities between all lineages of extant organisms, such as DNA, RNA, amino acids, and the lipid bilayer, give support to the theory of common descent. Phylogenetic analyses of protein sequences from various organisms produce similar trees of relationship between all organisms. The chirality of DNA, RNA, and amino acids is conserved across all known life. As there is no functional advantage to right- or left-handed molecular chirality, the simplest hypothesis is that the choice was made randomly by early organisms and passed on to all extant life through common descent. Further evidence for reconstructing ancestral lineages comes from junk DNA such as pseudogenes, "dead" genes that steadily accumulate mutations.


Sources

Mount DM. (2004). Bioinformatics: Sequence and Genome Analysis (2nd ed.). Cold Spring Harbor Laboratory Press: Cold Spring Harbor, NY.. ISBN 0-87969-608-7.

Douglas J. Futuyma (1998). Evolutionary Biology (3rd ed.). Sinauer Associates Inc.. pp. 108–110. ISBN 0-87893-189-9.

Haszprunar (1995). "The mollusca: Coelomate turbellarians or mesenchymate annelids?". In Taylor. Origin and evolutionary radiation of the Mollusca : centenary symposium of the Malacological Society of London. Oxford: Oxford Univ. Press. ISBN 0-19-854980-6.

Kozmik, Z; Daube, M; Frei, E; Norman, B; Kos, L; Dishaw, LJ; Noll, M; Piatigorsky, J (2003). "Role of Pax genes in eye evolution: A cnidarian PaxB gene uniting Pax2 and Pax6 functions". Developmental cell 5 (5): 773–85. doi:10.1016/S1534-5807(03)00325-3. PMID 14602077.

Kozmik, Z; Daube, Michael; Frei, Erich; Norman, Barbara; Kos, Lidia; Dishaw, Larry J.; Noll, Markus; Piatigorsky, Joram (2003). "Role of Pax Genes in Eye Evolution A Cnidarian PaxB Gene Uniting Pax2 and Pax6 Functions". Developmental Cell 5 (5): 773–785. doi:10.1016/S1534-5807(03)00325-3. PMID 14602077.

Land, M.F. and Nilsson, D.-E., Animal Eyes, Oxford University Press, Oxford (2002) ISBN 0-19-850968-5.

Chen FC, Li WH (2001). "Genomic Divergences between Humans and Other Hominoids and the Effective Population Size of the Common Ancestor of Humans and Chimpanzees". Am J Hum Genet. 68 (2): 444–56. doi:10.1086/318206. PMC 1235277. PMID 11170892.

Cooper GM, Brudno M, Green ED, Batzoglou S, Sidow A (2003). "Quantitative Estimates of Sequence Divergence for Comparative Analyses of Mammalian Genomes". Genome Res. 13 (5): 813–20. doi:10.1101/gr.1064503. PMC 430923. PMID 12727901.

The picture labeled "Human Chromosome 2 and its analogs in the apes" in the article Comparison of the Human and Great Ape Chromosomes as Evidence for Common Ancestry is literally a picture of a link in humans that links two separate chromosomes in the nonhuman apes creating a single chromosome in humans. Also, while the term originally referred to fossil evidence, this too is a trace from the past corresponding to some living beings that, when alive, physically embodied this link.

The New York Times report Still Evolving, Human Genes Tell New Story, based on A Map of Recent Positive Selection in the Human Genome, states the International HapMap Project is "providing the strongest evidence yet that humans are still evolving" and details some of that evidence.

"29+ Evidences for Macroevolution: The Scientific Case for Common Descent". Theobald, Douglas. Retrieved 2011-03-10.

"Converging Evidence for Evolution." Phylointelligence: Evolution for Everyone. Web. 26 Nov. 2010.

Petrov DA, Hartl DL (2000). "Pseudogene evolution and natural selection for a compact genome". J Hered. 91 (3): 221–7. doi:10.1093/jhered/91.3.221. PMID 10833048

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